1. Field of the Invention (Technical Field)
The present invention relates to apparatus for manufacturing composite materials of wood and plastic, particularly sheets of relatively high density wood-plastic composition (especially compositions incorporating ground, milled, or commutated whole trees such as juniper or pine), and related manufacturing methods.
2. Background Art
It is known to manufacture composite materials from mixtures of ground or milled wood and finely ground plastics. A method and apparatus for manufacturing such a composition is disclosed in U.S. Pat. No. 6,632,387 and U.S. Pat. No. 6,586,504, the disclosures of which are incorporated herein by reference. It is generally desirable to manufacture composite material in sheets or panels suitable for use in the construction of buildings, signs, and the like.
Current manufacturing techniques for thermoplastic-fiber composite materials focus on the extrusion process, whereby a molten cellulose/thermoplastic mix is forced under high pressure through a forming die. The product is shaped and formed corresponding to the die, and then allowed to cool. In some instances, the cooling of the extruded product is accomplished by wrapping the extruded product around one or more large, chilled, cylindrical mandrels. Such a cooling technique, however, flexes the product resulting in unacceptable fracturing of sheet or panel product having a thickness of more than about 0.5 inches. Alternatively or additionally, known fabrication processes require post-extrusion cooling mechanisms that are dozens of feet long, demanding large manufacturing facilities. Known extrusion-type production devices generally also are limited as to both the width and thickness of panels or sheets than can effectively be produced.
A further shortcoming of known systems and methods for manufacturing cellulose/plastic composites is the difficulty in cooling the product while also obtaining desirable product density. Upon expulsion from and extruder, product may expand, and any extensive further processing of the product, including cooling at atmospheric pressure, may result in a product of insufficient density and strength. Many composite products produced with known devices and methods lack desirable shear and bending strengths. Further, many composite products of current modes of fabrication are believed to be undesirably friable, compromising their integrity after prolonged exposure to weather extremes.
In summary, therefore, there is an unmet need for a method of manufacturing a wood-plastic composite material in sheets that can readily be cut into construction panels of conventional size (e.g. four feet by eight feet), which material is of adequate density and durability for use as a construction material. Such a fabrication method requires a combination of optimal pressure, and cooling, rates for an extruded material. Against this background, the present system and process was developed.